Modulation distortion correction



2 Sheets-Sheet 1 W. KUMMERER MODULATION DI STORTION CORRECTION OriginalFiled Oct. 31, 1933 1 R R E o w T N 1 N M R E M o 7 m K A w E Y W rm 91m LT Y m mm M 1 ccccomnn [3M 20 Hm 9 A F. CR F 5 fl 2o MODULA TINPOTENTIAL MODU LA TING POTENTIAL FREQUENCY CARRIEIZ/ Jan.'7,' 1941. w.KUMMERER MODULATION DIS'I'ORTION CORRECTION Original Filed Oct. :51,1933 2 Sheets-Sheet 2 CA RR/ER FREQUENCY MODULA TING-- POTENTIA LCARRIER FREQUENCY INVENTOR WILHELM KU MERE ATTORNEY POTENTIAL /-MODULATING division of my United States atented Jan. 7 1941 MonULA'rroNms'ron'rroN CORRECTION,"

Wilhelm Kummerer, Berlin, Germany, assignor to Telel'unken Gesellschaftfiir Drahtlose Telegraphic 111. b. H., Berlin, Germany, a corporation ofGermany Original application October 31, 1933, Serial No.

695,968. Divided and this application February 15, 1939, Serial No.256,444

4 Claims. British Patent #323,823 shows an arrangement wherein thedistortions caused by the curvatureof the tube characteristic and otherfactors (such as slighting" of certain frequencies) may a be compensatedby anti-regeneration (degeneration) coupling from the output circuit tothe input circuit of the said amplifier tube. By the aid of suchanti-regeneration, the gain or ampliflcation of the tube is diminished,yet it is feasible in this manner to straighten and at the same timeflatten the characteristic curve of the system, thereby diminishingdistortion. The distortion compensation justifies the loss in gaininvolved in many cases.

Now, when this known method is applied to valve or" tube transmitters,in other words, if anti-regenerative coupling by the modulatedoscillations is established from the output to the input, spuriousfeed-backs will be set up and these cause disturbing or interferingoscillations to be produced. It is known that radio frequency feed-back,upon reversal of phase, nearly always results in a feed-back at afrequency which is generally different from the working frequency withthe result that oscillations of this particular undesired frequency areproduced in the circuits and are hard to eliminate. These difficultiesarise also in the case of a circuit organization as disclosed in BritishPat. #172,389, which shows means to modulate by anti-regenerationproduced with modulated radio frequency oscil lations.

Another demerit residing in anti-regeneration 35 by means of themodulated radio frequency wave itself is that the radio frequency gridalternating voltage furnished from the preceding transmitter stage,because of anti-regeneration reducing the gain or amplification, must bekept at a higher 40 level so that, in the presence of grid" currentflow, the centre {power required for full uti lizationof thedegeneratively coupled tube is appreciable. i

Now, according to this invention, which is a application #695,968, filedOctober '31, 1933, all of the advantages described above can be had bymerely raising the modulating potential amplitude. The present inventionwhich also obviates the difficulties outlined above consists ineffecting degeneration by means of the audio frequency component or,optionally, also by means of the direct current component which areobtained by rectification of the modulated radio frequency oscillations,while the radio frequency is filtered out so that it cannot producespurious feed-back. In types of modulation in which, by means of varyingthe negative grid bias, the modulation curve can be statically recorded,for instance, in the case of direct grid current or grid voltage 5modulation, it is usually advisable not to suppress the direct potentialportion. This insures that fluctuations in the line voltage entaillesser variations in the antenna current. If, however,

the counter coupling is applied to the grid of a tube serving for theamplification of the modulated wave, fluctuations in the high frequencycurrent of the oscillatory circuit should not cause any staticdisplacement of the working point of the amplifier tube' and in thiscase only the alternating current component of the rectifier voltageisutilized for the countercoupling.

The novel features of my invention have been pointed out withparticularity in the claims appended hereto. 0

The nature of my invention and the mode of operation thereof will bebetter understood from the following detailed description thereof andtherefrom when read in connection with the drawings, throughout whichlike reference char- 5 acters indicate like parts, and in which:

Figures 1, 2, 3, 4 and 5 show various circuit arrangements including thefeatures of my novel invention. v

Figure 1 shows an embodiment of the invention by way of example for amodulating stage with grid voltage modulation. Here tube l is a highfrequency amplifier to which a high frequency grid potential of constantamplitudes is applied. .1 designates the rectifier tube coupled to themodulated wave output circuit of tube I, 3- is the modulation frequencytransformer to which the audio frequency alternating potential isapplied, and 4 is the direct current source supplying the required biasfor the grid of tube I. In the absence of my novel correction scheme andin the presence of normal modulation action the resultant modulationvoltage envelope of the carrier wave may be distorted. This distortion45 will be, apparent in a cathode ray oscillograph coupled to the outputof tube I. As pointed out more in detail hereinafter, the distortion maybe due to high iiiodulating potentials which swing the grid of tube I sothat the tube operates on 50 .the curved portion of its characteristiccurve,

its. characteristic. This non-linearity may result in the peaks of themodulation envelope or the carrier wave being extended or shortened. The

countercoupled direct current voltage supplied voltage modulationaccording to the known Heisingmethod is accomplished. Here, as in Fig.1, l is the high frequency amplifier, to which I a high frequency gridvoltage of constant amplitude is applied through the coupling coil 2. 5represents a modulating tube. tube 5 is negatively biased by means ofcurrent source 4 and through transformer 3 there is sup-" plied theretothe alternating audio frequency potential serving for modulation. Aplate current source 6 for amplifier l and modulator 5 is connected asshown between the anode and cathode of said tubes. I is the rectifiertube for the countercoupling circuit coupled to the output circuit ofthe high frequency oscillator. The alternating audio frequency voltageacross the terminals of the condenser 8 at the utput of therectifierwhen modulatingpotentia s are im-= pressed on the carrier issupplied to th egrid circult of the modulating tube 5 by means of thetransformer 9 so that countercoupling is produced. Blocking condenser lblocks the direct current voltage out of the primary winding of thetransformer 9.

Between the modulatedstage l and the antenna circuit A one or severalhigh frequency amplifier stages may be connected in the known manner,and the countercoupling to the modulated stage may be carried out fromany desired amplifier stage.

The counter coupling reduces the existing distortions to a fraction ofthe original ones. A complete linearity cannot be obtained throughcountercoupling. In order to obtain an essential reduction of thedistortions, the. degree of the counter coupling, i. e. the proportionbetween the countercoupling voltage and the voltage required formodulation must be chosen large, however this increase of thecountercoupling must not be extended too far else difficulties indimensioning will be encountered. Iithe hereby obtained reductionofdistortion is insufficient, the action of the countercoupling can beessentially augmented by means of inserting between the high frequencystage for which distortions are to be reduced, and the linear rectifier,a high frequency amplifier stage in which the amplification takes anon-linear course. A

Figure 3 shows an embodiment of this idea by way of example, in atransmitter modulated according to the principle of grid potentialmodulation. Here I is the high frequency amplifier of transmitterstage,which is to be modulated, 20 designates the amplifier tube of the highfrequency amplifier stage connected between the high frequency amplifierl and the rectifier. 3 is the rectifier, tube. Otherwise the arrangementcorresponds with the circuit of Figure 1.

The counteror differential coupling reduces existing distortions to afraction 'oftheir original value. However, perfect linearity isunattainable in this manner because the operation of acountercouplingscheme is based uponthe presence of modulation potentialamplitudes which extend into the curved portion of the modulation Thegrid of the aim of the present circuit scheme.

characteristic, whereby flattening of the peaks of the modulated radiofrequency alternating potential occurs because of this very curvature.Because of the countercoupling of the rectified modulated radiofrequency distorted in this man ner, it will be noted that a correctionvoltage lower than the voltage from which it is produced is caused tobecome operative in a sense opposite to the sense of the modulatingpotentials'at the instance when the curvature'of the modulationcharacteristic is reached, with the result that the modulation voltageis distorted. Thus, a less distorted modulated radio frequency voltagearises in the output circuit of the radio frequency circuit of stage I.Hence, thevoltage used for the countercoupling must always be distorted.Inasmuch as according to the described methods it is obtained from themodulated radio frequency energy existing in the output circuit of tubeI, the same normally will be distorted and this is The more correctionof distortion obtained by the countercoupling, the higher thecountercoupled correction voltage used must be. The modulation voltagewill also have to be higher in order thatan adequate grid modulationvoltage may be secured in spite of counter action of the countercoupledcorrecting potentials. Hence, complete correction of distortion will beattainable only in the presence of infinitely high amplitudes of themodulation potential-as well as of the countercoupled voltage acting inopposition. Practical limits are imposed. upon the correction of dis--tained by the aid of a distinct amplifier stage having anon-linearcharacteristic, the characteristic of the said stage being similar inform to the characteristic of the transmitter. In such an arrangement aswill be seen, a distorted voltage for countercoupling and differentialpurposes will be available-when a perfect voltage free of distortion isavailable in the radio frequency amplifier stage I, due to thedistorting effect of the countercoupling. In fact, it would be feasiblein this manner to accomplish overcompensation.

In using the countercoupling of the present invention it is obvious thatthe alternating voltage serving for the modulation-must be increased bythe value of the countercoupling voltage. In case of intensivecountercoupling this is often held-to be detrimental. It is possible toavoid this drawback by applying the modulating voltage in opposition tothe correcting voltage furnished by the rectifier to thecountercoupling. This may be done by means of a bridging arrangementknown as such, so that in case of a. modulating curve the course ofwhich follows a straight line jand in the presence of proper phaserelation betweenthe voltages, the voltage supplied by the rectifier andthe modulating voltage to the bridge circuit eliminate each other- Thisleaves only the voltages produced by distortions in the modulation,process utilized for reducing distortions.

Figure 4 shows an embodiment of the modifica- 7 static countercouplingtakes place.

Such coupling is often desired in view of reduction of. the steepness ofthe modulation curve and the hereby entailed greater stability of theplate current at fluctuations of the operating means and of the antenna.Thus the circuits according to Figures 3 and '4 are combined in suchmanner that on the one hand a countercoupling to the input circuit oftube i occurs across the transformer I! in series with transformer 3also connected to the corners of bridge II as in Fig. '4, while on theother hand the resistance passed by the output current of rectifier I,by which static countercoupling is effected, is connected in series asin Fig. 3 with the grid bias source 401 tube I. An amplifying tube ATmay be included in the circuit between the arm of i3 and the primary ofi2. Voltage limiting means VL may be coupled across the output of AT.

Having thus dmcribed my invention operation thereof, what I claim is:

1. Signalling means comprising, a thermionic relay tube having input andoutput electrodes and input and output radio-frequency circuitsconnected therewith, a source of oscillations of carrier frequencyconnected with said input circuit, a source of modulating potentials,two modulation potential impedances connected in series between a pairof electrodes in said tube, means for impressing modulating potentialsfrom said source and the of modulating potentials on one of saidimpedances, a bridge circuit. said bridge circuit having pairs of nodalpoints, 'a circuit connecting said one of said impedanc'es to one ofsaid pairs of nodal points, a rectifier coupling another pair of saidnodal points to the output circuit of said relay tube, and a circuitcoupling an arm of said bridge circuit to the other of said impedances.

2. In a modulation system an electron discharge device having inputelectrodes and output electrodes coupled in alternating current circuitsin which we've energy to be modulated is caused to flow. a source ofmodulating potentials, two modulation potential impedances connected inseries between the input electrodes of said discharge device, means forimpressing modulating potentials from said source of modulatingpotentials on 7 one of said impedances, and means for improving thelinearity of said modulation comprising a bridge circuit having pairs ofnodal points connected by impedance arms, rectifier means havingelectrodes excited by wave energy from the output electrodes of saiddevice, a resistance connected with said rectifier electrodes, meanscoupling said resistance to a pair of said nodal points,

means for impressing modulating potentials on another pair of said nodalpoints and means for impressing potentials from one of the impedancearms of said bridge on the other of said modulation potentialimpedances.

3. A system as recited in claim 1 wherein said last named means includespotential amplifying means. 7

4. Signalling means comprising, a thermionic relay tube having input andoutput electrodes coupled in alternating-current circuits in which waveenergy to be modulated is caused to flow, a

source of modulating potentials, two modulation means connecting an armof said bridge to the other of said modulation potential impedances, andmeans coupling electrodes of said rectifier in shunt to said resistance.

WILHEIM KUMMERER.

